The present invention relates to catalyst systems tailored to remove sulfur and heavy metals from a hydrocarbon feedstock and a process using these systems. The systems are in general terms fixed bed catalyst systems. More particularly, the catalyst systems comprise at least two layers of catalyst particles. We characterize the first layer as having a relatively high desulfurization (HDS) activity compared to the second layer and as having a relatively small average macropore diameter compared to the second layer. Typically, these layers comprise the demetalation (HDM) catalysts which protect a high activity residual oil desulfurization catalyst from premature deactivation by metals deposition. The process which uses these catalyst systems comprises passing a hydrocarbon feedstock containing sulfur and heavy metals over the system at hydrometalation and hydrodesulfurization conditions.
Most heavy crudes contain significant amounts of sulfur and heavy metals. Heavy metals such as nickel and vanadium create problems for refiners by depositing within the catalyst particles. As a result, they block the catalyst pores, and deactivate the catalyst. Workers in the field have proposed a variety of schemes to remove heavy metals from petroleum feedstocks.
One approach is to frequently replace the fouled catalyst, but this is wasteful and results in costly under-utilization of the catalyst. In recent years, workers in the field have developed hydrometalation catalysts to protect the more active hydrodesulfurization, hydrodentifrication, and/or hydrocracking catalysts. Schemes of layering varieties of catalysts which differ in pore size, support composition, and metals capacity can result in more efficient use of the individual catalysts.
Conventional processes which remove nickel and vanadium generally have increasing HDS activity, decreasing macroporosity, decreasing average macropore size, and/or decreasing average mesopore size, along the direction of feed flow through the layered bed. We define the term "macropore" to mean catalyst pores or channels or openings in the catalyst particles greater than about 1000 .ANG. in diameter as measured by mercury intrusion. These pores are generally irregular in shape and pore diameters are used to give an approximation of the size of the pore openings. The term "mesopore" is used herein to mean pores having an opening of less than 1000 .ANG. in diameter. Mesopores are, however, usually within the range of 10-300 .ANG. in diameter. We use the term "metals capacity" herein to mean the amount of metals which can be retained by the catalyst under standard demetalation conditions.
Previous workers in the field found macroporosity and the presence of larger mesopores to be strongly related to the capacity of catalyst particles to retain metals removed from a hydrocarbon feed contaminated with nickel and vandium. In the downstream catalyst zones, they prefer predominantly mesoporous catalysts. They found them to have substantially higher catalytic activity for HDS compared to catalysts having lower surface areas and a substantial macroporous structure.
For example, U.S. Pat. No. 3,696,027 to A. G. Bridge, issued Oct. 3, 1982, suggests sequentially contacting the feedstream with a graded system comprising three fixed beds of catalysts having decreasing macroporosity along the normal direction of feed flow. In order to lengthen the HDS run, the catalyst particles of the first bed have at least 30 volume percent macropores; the catalyst particles of the second bed have between 5 and 50 volume percent macropores; and the catalyst particles of third bed have less than 5 volume percent macropores. Bridge also teaches that the three fixed beds have progressively more active HDS catalysts along the direction of hydrocarbon flow. The third catalyst bed (which contains the most active HDS catalyst) contains high surface area particles having an average pore diameter of at least 50 .ANG., preferably at least 80 .ANG., and more preferably at least 100 .ANG..
Unexpectedly, we have discovered that by "reverse-grading" at least part of the HDM catalyst system, we can significantly increase the cycle life of the entire catalyst system. We use the phrase "reverse-graded" system to connote two or more catalyst layers in which at least two successive layers have decreasing HDS activity and increasing average macropore diameter along the direction of hydrocarbon flow. This is in contrast to the usual grading with increasing activity and/or decreasing average macropore diameter along the direction of hydrocarbon flow. By using such an HDM system, we are able to increase sulfur removal and metals removal in the HDM catalyst system. This allows us to increase the amount of HDM catalysts in the entire catalyst system, thereby increasing the metals capacity of the entire catalyst system and extending the life of the catalyst system. Accordingly, it is the primary object of this invention to provide reverse-graded HDM catalyst systems which significantly increase the cycle life of the entire catalyst system.